CHAPTER XII 



ORE DEPOSITS 



Ores are concentrations in the earth's crust of economically valu- 

 able minerals. 



Ores in Ready-made Cavities. — A common form of deposit is 

 the vein, or the filling of a fissure in a rock. The contents of a fissure 

 may consist partly or wholly of minerals, some of which may or may 

 not be of economic value. When mineral veins contain ores, they are 

 called lodes by miners. Fissures and other cavities are formed in 

 several ways, as has been seen (p. 262). (1) Stretching movements 

 of the earth's crust fracture it, producing open cracks ; (2) faulting 

 (p. 261) forms fissures and brecciated zones ; (3) fissures are developed 

 by shrinkage, such as occurs when igneous rocks cool or when 

 limestone is changed to dolomite ; (4) the joints of rocks are widened ; 

 (5) cavities are formed in limestone by solution. Cavities formed 

 in any of these ways may contain ores. 



Fissure Deposits. — Metalliferous veins are not composed entirely 

 of metalliferous minerals, but on the contrary the latter often 



constitute a very small percent- 

 age or the vein ruling. Ihe use- 

 less vein material is called gangue, 

 the common gangue minerals 

 being quartz, calcite, and fluorite. 

 In some veins the contents are 

 arranged in bands parallel to the 

 walls, the minerals and ores of 

 one wall being represented by 

 corresponding bands on the op- 

 posite wall (Fig. 356). This 

 arrangement is the result of the deposition of minerals from solution 

 on the two walls of the fissure at the same time. Such a symmetrical 

 arrangement, however, is not common, the layers usually being 

 thicker on one wall than on the other, while frequently a layer on 



37o 



FlG. 356. Handed veins: A and Z), 

 quart/; B, sphalerite; C, galena. 



